April 7, 2022

SKALE Tech Deep Dive: IMA Bridge FAQ

Chadwick Strange
VP of Product

What is a network bridge or inter messaging agent (IMA) or and why is it important?

Network bridges are essential components between connected blockchain solutions as they allow for the secure transfer of digital assets between networks. These bridges serve as transfer agents for the assets. Instead of transferring the actual assets, however, they serve as a combination of lockboxes and proxy providers to provide verified transfer of digital equivalents in combination with locking or unlocking the originating tokens.

How do network bridges/IMAs work?

On a basic level, a bridge lets a user send an arbitrary message from one chain to another chain in such a way as to allow the contents of that message to become functional in that second chain without risk of double spend or separate concurrent activity on both chains. This message passing implies a locking or unlocking of the contents of the messages (i.e. the tokens) in one chain and the provisioning of a synthetic asset on the other chain.

There are a number of approaches to network bridges. Many of these suffer deficiencies in some form or other which impact adoption and user experience. These deficiencies include:

  • Overly long transfer exits – hours, days, or even up to a week.
  • Inflexibility – many only support ERC20 tokens, others require filling out a form and waiting for the dev team to approve your desired token type
  • Custody risks – On some L2s, the custody of tokens minted on that network do not remain with the owner after exiting to the L1 (the bridge owner mints the NFT in its name)

You can also add complexity, potentially faulty security mechanisms, and lack of customization to complete the picture of immature bridging technologies.

How does the SKALE IMA Bridge work?

The SKALE IMA Bridge works in a simple, transparent, and highly secure manner and sets a standard in the way that network bridges should work. Token holders signal their intention to transfer tokens and other digital assets from Ethereum to a SKALE Chain (or from the SKALE Chain to Ethereum). When sufficient validators for the receiving SKALE Chain see this transfer signal on chain and validate it as well-intentioned and true, the transfer initiates on the Ethereum mainnet (or SKALE Network if in reverse).

The assets are secured and placed into a deposit box on the Ethereum mainnet via IMA contracts on the mainnet. After a set number of blocks (currently 10) have passed on the Ethereum mainnet – so as to ensure transactions are fully recognized and valid – a transfer request gets sent to a proxy called the SKALE IMA Agent. This proxy runs as a container in a SKALE Node and calls the SKALE TokenManager operating within the SKALE Network to process this transfer within the respective SKALE Chain. As opposed to days or even hours, this transfer happens quickly, buffered only by Ethereum mainnet block creation.

A transfer in the other direction, going from a SKALE Chain to Ethereum, happens in a similar manner, although in the case of exiting SKALE into the Ethereum mainnet, the tokens are burned on the SKALE chain and unlocked on the mainnet. In the case of tokens that are minted on the SKALE Chain and then transferred to Ethereum – a minting operation on Ethereum takes place as part of the transfer in such a way as to preserve the ownership of the minted token.

What security mechanism is used to lock tokens on the Ethereum mainnet and synthesize tokens on SKALE (or burn on SKALE and release on Eth mainnet if going in the other direction)?

The SKALE IMA Bridge makes use of the combination of SKALE’s operational ties to the Ethereum network, its Proof of Stake and ABBA consensus models, and its foundation in BLS cryptography to provide a secure, fast, and economic way to transfer a rich set of assets to and from between the Ethereum mainnet and SKALE Chains.

The DepositBox contract runs on the Ethereum mainnet as does the contract that handles assets returning to the mainnet. When sufficient validators of SKALE Chains notice a transfer request and validate it as legitimate and authorized, the process of depositing on the Ethereum mainnet and synthesis of equivalent tokens on the SKALE Chain is effectuated. Transfer requests get sent to a proxy called the SKALE IMA Agent. This proxy in turns calls the SKALE TokenManager operating within the SKALE Network to process this transfer within the respective SKALE Chain. Ten (10) blocks must pass on the Ethereum mainnet prior to the initiation of the transfer.

What type of tokens does the SKALE IMA Bridge support?

The SKALE IMA Bridge offers full support of ETH, ERC20, ERC721, and ERC1155 token standards and has built-in functionality to support custom tokens. This support is on a chain-by-chain basis. Chain owners can decide what tokens to accept – by standard or by token issuer, for example – in the event that chain owners want to confine token transfers to specific types so as to simplify asset management or provide protective measures against inadvertent transfers.

Is the SKALE IMA Bridge customizable?

Yes. The SKALE IMA Bridge allows extensive token support that can be customized to suit the needs of a particular SKALE Chain. The SKALE IMA Bridge contract also has hooks so that chain owners can specify new token types or other forms of digital assets that they want to support. This level of customization is difficult to find in other network bridges.

How long does it take transfers to settle?

Transfer settlements take minutes – gated only by validators recognizing an onchain transfer request and the passage of 10 blocks. (The passage of 10 blocks is to ensure transactions are fully recognized and valid.) This quick transfer speed differs from many other bridge approaches. Plasma-like bridges, for example, may have exit periods that might last up to 1-2 weeks (giving other parties the opportunity to contest an exit).

How much does it cost to use the SKALE IMA Bridge?

SKALE Chain to SKALE Chain transfers have zero costs associated with them.

The only costs incurred are from Mainnet operations but these have been optimized by the SKALE team to be as economical as possible (equivalent to a Uniswap transaction in many cases). Mainnet fees are unavoidable when bridging to and from Ethereum. In the case of SKALE, they are as minimal as can b,e as the Proof of Stake nature of the SKALE Network and its economic model (chains are sponsored by dapps, protocols, DAOs, and other entities) means that SKALE chain transactions have little to no costs associated with them.

Why is there the need to fund exits from a SKALE Chain to Ethereum in excess of the estimated Ethereum fees?

The cost of moving from a SKALE Chain to the Ethereum mainnet is the same or less than other bridges to other Layer 2 networks.

An issue arises, however, because of the combination of Ethereum gas price volatility and the need to fund exits in advance of the transaction. In order to address this situation, a buffer is added to the estimated cost of the exit transaction. The funding amount – which consists of the estimate plus buffer – equates to the approximate cost of three exits. The number three doesn’t have any significance aside from it being the amount deemed more than sufficient to account for Ethereum’s transaction volatility.

Excluding mass exits or surge conditions, the exit transaction fees will almost always be at or near the estimated amount and so when the transaction goes through and is verified, any excess funds will remain as a credit in the user’s current account in the community pool. This amount can remain there for future exits or can be remitted back to the sending account used in the transaction. The community pool is the name given to the component that funds exits in the SKALE IMA Bridge. Funds are not actually pooled per se as any funds you contribute are in self custody, retained by you, and used to fund only those exit transactions performed by you.

What’s just been described is the default exit flow, and whether this exit flow is used is at the discretion of the SKALE chain owner.  What’s great about SKALE is its flexibility. The SKALE chain owner can also choose to subsidize exit fees from a specific wallet or other mechanism.

Is there any markup in fees when using the SKALE IMA Bridge.

No. The cost of entry and exit from a SKALE Chain is the true cost of the Ethereum network fees without any markup, SKALE Network fees, or SKALE Chain owner fees.

Are there any gas transaction fees for SKALE Chain to SKALE Chain transfers?

No. Transfers from SKALE Chains to SKALE Chains incur no gas transaction fees. Transfer costs are only incurred when moving to and from the Ethereum mainnet.

Are their protections against mass exits?

Yes. In order to ensure a timely and fair exit process – one that works for everyone in the network – as well as to reduce backups caused by mass exits, user's exit transactions are rate limited.

What algorithms and cryptography are used for the SKALE IMA Bridge?

The SKALE IMA Bridge makes use of BLS cryptography, Ethereum mainnet smart contracts, Proof of Stake and ABBA consensus models, and other unique aspects of its design to allow for a fast, secure, and cost efficient transfers to and from SKALE Chains and the Ethereum mainnet.

Is custody of tokens preserved when transferring tokens from SKALE to the Eth mainnet (and why should this even be an issue)?

Yes. The SKALE IMA Bridge preserves custody of tokens that are minted on a SKALE Chain when they exit to the Ethereum mainnet, a claim that many bridges cannot make. Some networks impair custody by minted tokens on the mainnet under their auspices necessitating a transfer operation to the rightful owner.

Preservation of capability is a must-have for NFT-based dapps, along with any other dapp that might mint a token, in that a primary reason for using a scalability solution is to avoid high-cost minting operations on the Ethereum mainnet. Using a network bridge that does not preserve custody introduces unnecessary and potentially problematic counter-party risk to any and all exits from that network. The SKALE IMA Bridge preserves custody and eliminates this risk.

Is the SKALE IMA Bridge fault-tolerant with respect to node outages?

Yes. The Proof of Stake consensus model in combination with the unique ABBA consensus algorithm allows a certain number of nodes to become inoperable without affecting the chain’s ability to operate in a secure manner. This capability has been demonstrated by SKALE via mathematical proofs.

The algorithmic approach that SKALE uses allows any node to propose blocks thereby necessitating only a sufficient number of other nodes to validate blocks so as to achieve consensus. Of course, in the event of an outage in any corresponding network in a particular exchange, the transfer would be dependent on the operational integrity of the other network.

How does the SKALE IMA Bridge differ from Arbitrium or Optimism bridges?

The SKALE bridge works in a different manner than either Arbitrium and Optimism and the approaches are more difficult and technical to compare. The performance metrics are dramatically different and easier to discuss.

The main difference is Arbitrium or Optimism can take up to a week to finalize exit from the networks whereas the exit from SKALE is finalized within 1 minute. The time to enter their networks can range from 10-20 minutes, whereas SKALE entry takes 1 minute.

Arbitrium or Optimism also don’t support ERC-721 and ERC-1155 tokens nor do they offer customization as does SKALE.

As far as entry and exit costs. Entry cost is about the same between SKALE and Arbitrium. Arbitrium costs slightly less than SKALE for exit. Optimism costs more to enter as well as almost 75% more than SKALE to exit.

How does the SKALE IMA Bridge differ from the Polygon Bridge?

The biggest difference is the Polygon bridge does not support ERC-721 and ERC-1155 tokens. Given the size and growth of the NFT market, this lack of support presents a severe limitation.

SKALE not only supports NFT standards such as ERC-721 and ERC-1155 but also provides rich customization to turn on or turn off token support as well as to allow for custom hooks within exit contracts. The SKALE IMA Bridge also preserves custody of tokens minted on a SKALE Chain but exited to the mainnnet. Most other bridges cannot make this claim and pose problems when NFTs are minted within their networks.

Costs are within range of each other although recent benchmarks did show SKALE to cost 30% less.

What sets the SKALE IMA Bridge apart from other network bridges?

One of the big differences is the speed of entry and exit is much faster with the SKALE IMA Bridge than any other bridge. SKALE entries and exits take approximately 1 minute. Other bridges take minutes to hours to enter and take hours to up to a week to exit. Long exits are largely unworkable in a production environment, even with liquidity pools and staking.

Another major difference is token support. The SKALE IMA Bridge supports ERC-20, ERC-721 and ERC-1115 whereas most others only support ERC-20. Given the size and growth of the NFT market, this lack of support from other bridges presents a severe limitation.

From a cost perspective, SKALE costs less enter or exit or is on par with almost all other bridges in production. SKALE may require a bit more of a deposit for exit but that is rebated into the users custody post transaction for reasons detailed above.

The SKALE IMA Bridge makes use of the combination of SKALE’s operational ties to the Ethereum network, its Proof of Stake and ABBA consensus models, and its foundation in BLS cryptography to provide a secure, fast, and economic way to transfer a rich set of assets to and from between the Ethereum mainnet and SKALE Chains.


SKALE is an Ethereum native multi-chain network composed of an unlimited number of secure, decentralized, high-performance blockchains for bringing NFTs, DeFi, and Web3 to billions of users. SKALE’s highly configurable platform was built to support an ever-expanding set of Dapp-specific chains that run without centralized dependencies. Plus, SKALE’s unique pooled security system and containerized node architecture enable developers to deliver a high-speed, seamless user experience without gas fees or latency.

SKALE’s open-source platform provides quick consensus with fast block times and instant finality and includes EVM compatibility, on-chain NFT file storage, rollup contract execution, zero-cost minting, and gasless transactions, and machine learning smart contract functionality.

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